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Controlling Algae#013
March 14, 2010

Welcome from the Koi Pond Guide to the 53 new subscribers.

Controlling Algae
Green water is caused by a bloom of microscopic algae that contain the green photosynthetic pigment, chlorophyll. Pond water containing these microscopic algae is usually free of both ammonia and nitrite, quite simply because these tiny organisms thrive on the final breakdown product of the nitrogen cycle, namely, nitrate.

Nitrate is a powerful fertilizer for plants, and so as the filter unit becomes established and the quantity of nitrate in the pond increases, the algae thrive in the abundance of this nutrient.

Green water is not harmful to Koi, but positively beneficial; Koi feed on the tiny animals that eat the algae and in this way green water is one of the best color enhancers of Koi available.

In Japan, many Koi enthusiasts keep their Koi at home in their ponds through the winter months but return them to the farms during the summer so that the Koi can benefit from spending several months in green water.

The disadvantage of green water, of course, is that you are unable to see your Koi. Not only will this spoil your enjoyment, but Koi that have injured themselves or are suffering from parasites, such as anchor worm, will remain hidden from your attention.

Green water can be very persistent and may take many months to clear from a newly established pond. The water may periodically go green in an established pond, depending on the aspect of the pond and the climatic conditions. During hot, sultry spells, algae will use essential oxygen at night, it is worth running an external pump, such as an aquarium pump with an airline and airstones in the pond to ensure maximum aeration of the water.


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Some Koi Keepers have started to use ultraviolet (UV) light as a means of controlling algae. (UV light is very harmful; never remove the bulb from its protective casing while the light is switched on.) UV light damages the tiny algal cells internally, and thus kills them. The UV unit is usually placed at the end of the filter system, before the water is returned to the pond.

One of the major myths concerning this form of light is its ability to kill bacteria and render the pond sterile. Although this is true in part, the water passing through the unit would have to be almost motionless for the UV light to kill most of the bacterial. In practice, the flow of pond water through an UV light system is too fast to seriously reduce the number of bacteria present in the pond water. Be sure to replace the UV tube every six to nine months if you are using it continuously; after this period it ceases to be effective even though it appears undimmed.

Chemical Products

Many chemical products are available to clear algae from ponds. When using these algicides, be sure to follow the manufacturers directions and never exceed the recommended dosage or frequency of treatments. There is always the possibility that, used incorrectly, such chemicals can harm the fish and aquatic plants. And remember that a sudden die back of dense concentrations of algal cells can cause water quality problems. In such cases, consider using some mechanical method of removing the bulk of the algae, such as high-rate mechanical filters, before dosing the water.

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